Introduction to Flask

Flask is a lightweight and powerful web framework for Python. It is designed to make getting started quick and easy, with the ability to scale up to complex applications. Flask is often referred to as a micro-framework because it provides the essentials without imposing too many constraints on the developer.

Key Concepts

1. WSGI (Web Server Gateway Interface)

WSGI is a standard interface between web servers and web applications in Python. Flask uses WSGI to handle incoming HTTP requests and send responses back to the client. This standardization allows Flask applications to run on various web servers, making it highly flexible.

2. Routing

Routing in Flask involves mapping URLs to specific functions in your application. This allows you to create different pages or endpoints that users can access. For example, a URL like /about can be mapped to a function that returns information about the website.

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Welcome to the Home Page!"

@app.route('/about')
def about():
    return "This is the About Page."
    

3. Templating

Flask uses Jinja2 as its templating engine. Templating allows you to create dynamic HTML pages by embedding Python code within HTML. This makes it easier to generate content that varies based on user input or other conditions.

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/greet/<name>')
def greet(name):
    return render_template('greet.html', name=name)
    

4. Request Handling

Flask provides tools to handle different types of HTTP requests (GET, POST, etc.) and access data sent by the client. This is crucial for building interactive web applications where users can submit forms or interact with the server.

from flask import Flask, request

app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    data = request.form['data']
    return f"You submitted: {data}"
    

5. Extensions

Flask is highly extensible through its ecosystem of extensions. These extensions add functionality such as database integration, authentication, and more. For example, Flask-SQLAlchemy simplifies working with databases, while Flask-Login handles user authentication.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///mydatabase.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)

@app.route('/user/<username>')
def show_user(username):
    user = User.query.filter_by(username=username).first()
    return f"User: {user.username}"
    

By understanding these key concepts, you can start building powerful and flexible web applications with Flask. Each concept builds on the previous one, allowing you to create more complex and feature-rich applications as you progress.